Telephone lines not only function to convey voice signals but are now widely used for data transmission in conjunction with teleprinters, computers and other data processing or terminal equipment. When trouble is encountered in the use of customer equipment connected to a telephone line, the cause of the trouble is not readily apparent. The telephone line may be the source of problem and, therefore, must be tested by itself. This testing generally requires a telephone repairman at the customer's premises in order to physically disconnect the customer equipment and insert a known test impedance across the line for test purposes. If the line is found to be in proper operating order the fault lies in the customer's equipment and if such equipment is owned by the customer it is his responsibility to make necessary repairs.
In the event operating trouble developes in the customer's equipment it would be highly desirable if the telephone company could determine the location of the trouble without sending a repair crew to the customer's premises and without disturbing the customer. The following U.S. Pat. Nos. disclose systems directed toward this end: 3,739,107, issued June 12, 1973 to Lucian W. Spencer, 3,843,848, issued Oct. 22, 1974 to Russel G. Cox, 3,912,882, issued Oct. 14, 1975 to Ronald H. Beerbaum.
In each of the arrangements disclosed in the above patents, power for operation of the loop back system is obtained from the customer's premises or from a secondary battery charged from loop power and connected to the system at all times. The customer pays for the power in the former case and in the latter case the battery imposes service requirements. Many customers find such systems to be objectionable and, consequently, such systems have not met with general acceptance.
In applicant's system, operating power is obtained from the 48 volt battery normally available at the central office. The system does not require an auxiliary battery or any other component that could require replacement after a period of time.
U.S. Pat. No. 3,636,280, issued Jan. 18, 1972 to Louis E. Wetzel, discloses an arrangement in which a 130 volt battery provides power for operation of a loop back circuit. This battery is available in some central offices for coin telephone station operation. However, operation from this 130 volt d.c. source presents the following problems:
1 -many central offices do not have the 130 volts available, PA1 2 -the high voltage applied to the subscriber's loop presents a hazard, and PA1 3 -a metallic path must exist from the test position in the central office to the subscriber's location.